Inhibiting formation of natural gas hydrates



Dec. 23; 1941. B. MILLER 6 INHIBITING FORMATION OF NATURAL GAS HYDRATES' Filed April 25, 1936 PKFSSUIfE raw MA /r LI um LEVEL A 221245aux; 24

@115 25g I [fa/FEED REULATaf 643 LINE INVENTOR I .BENJ/IMM/ MILLER BYATTORNEY than the freezing temperature of water.

Patented Dec. 23, 1941 ArsNr OFFICE INHIBITIN G FORMATION OF NATURAL GASHYDRATES Benjamin Miller, Richmond Hill, N. Y., assignor to CitiesService Oil Company, New York, N. Y., a corporation of PennsylvaniaApplication April 25, 1936, Serial No. 76,486

11 Claims. (Cl. 48-190) This invention relates to a process forinhibiting formation of solid hydrates in high pressure natural gastransmission lines during cold weather.

Natural gas transmission pipe lines are usually buried in the groundbelow the frost line so that difdculty is seldomencountered fromformation of ice obstructions in the line during cold weather.Nevertheless the gas carrying capacity of such lines is often seriouslyreduced during cold weather owing to the formation of deposits of solidhydrates of the normally gaseous hydrocarbon components of the gas. Atthe high pressures which are usually maintained in such transmissionlines formation of solid hydrates 15 may occur at temperatures severaldegrees higher The highest temperature at which solid gas hydrates canexist in any particular transmission line depends on the pressure of thegas, the composition of the gas, and the concentration of water vapor inthe gas. The exact composition of the hydrates is difflcult to determineand of course varies as the composition of the gas varies.

The principal part of the solid deposits is probably the hexahydrate ofmethane.

The primary object of the present invention is to provide aneconomically practicable and eiflcient method for inhibiting formationof solid gas hydrate in natural gas transmission lines during coldweather. Since one of the principal factors governing formation of solidgas hydrate is that of pressure, one possiblevmethod of preventing theirformation would bto operate the line at a presproblem, however, becausethe transmission line must normally be operated at a higher pressurethan the hydrate decomposition pressure-in order to carry the volume ofgas required by its rated capacity. An alternative method would be toreduce the partial pressure of water vapor in the gas entering the linebelow the water vapor pressure of the hydrates normally formed at theminimum operating temperature and maximum operating pressure of theline. This method,

while thoroughly practical, requires a large capital investment whichcannot always be justified.

Essentially the process forming the subject of the present inventioncontemplates inhibiting formation of gaseous hydrocarbon hydrates in adensation from the gas at the points of minimum temperature obtaining inthe line of a liquid water solution. This result is effected by addingto the gas flowing through the line at a point 5 on the up-stream sideof the point of minimum temperature, one or more of a group of volatileand water soluble fluids in amount sumcient to form a condensate withwater which does not combine with the gaseous hydrocarbons to form solidhydrates at the mini-mum temperature and maximum operating pressure ofthe line. Among the fluids which are suitable for this purpose,monohydric aliphatic alcohols in the range from methanol through amylalcohol are preferred because they may be added without injuriouseffects on materials which the gas may contact;

although the corresponding aliphatic ketones and acids and certain gasesincluding carbon dioxide and ammonia may also be used. Secondary butylalcohol is a preferred reagent, because it has a vapor pressure notgreatly different from that of water. However, from a practical andeconomical standpoint methanol at present seems to be the best reagentof the group indicated, with ethanol as a probable second choice.-preferred plan of operation is to continuously introduce into the gasline during Fall and Winter a small measured proportion of the volatilewater soluble fluid reagent in amount suflicient to prevent solidhydrate formation.

With the above and other objects and features in view, theinvention'will be hereinafter more particularly described by referenceto the accompanying diagrammatic illustration of a preferred type ofequipment for introducing the fluid reagent into the natural gastransmission line.

Referring to the drawing, Ill indicates a small a broken section of anatural gas transmission line located at a point on the high pressureorupstream side of any point in the line where deposition of gas hydratesmight occur. I2 indicates a supply tank for a volatile watersolublefluid reagent to be used for inhibiting formation of natural gashydrates in line l0. Injection of the fluid reagent into the line wouldnormally 1 be made at the compressor station, downstream from theaftercoolers, preferably downstream from the first drip. In the case oflines having several compressor stations, injection might be made ateach station, or at alternate stations For example, tankl! mayberconsidercd as a source of supply for methanol. I4 is a combinedpressure-gravity feed tank to which the methanol natural gastransmission line by effecting conis conducted by a valved pipe l6 fromtank l2.

The'

Tank I4 is provided with a gauge glass Hi to enable the operator toobserve the level of methanol in tank l4. Methanol is fed (preferably ata continuous controlled rate) from tank l4 into line I through a pipeequipped with a check valve 22. On the high pressure (upstream) side ofthe point of connection of feed proportion the rate of feed of methanolto line H1 in accordance with the metered rate of flow of natural gasthrough line l0.

. When a volatile water' soluble fluid reagent such as methanol is addedto the natural gas flowing through line It at a rate not substantiallyin excess of that necessary to inhibit hydrate formation at a more orless remote point in the line, under normal conditions of temperatureobtaining in the line the added alcohol may all be vaporized.v As thetemperature drops further along the line, condensation of a solution ofwater in alcohol will occur. The water reduces the alcohol vaporpressure, and the alcohol reduces the water vapor pressure, so that morewater is condensed than if the alcohol were not present.

The quantity of alcohol in the condensate is proportional to thequantity of water in the condensate.

Knowing the minimum temperature occurring in the line and the maximumpressure at which it is necessary to operate the line, the amount ofalcohol which must be added to a definite volume of natural gas flowingthrough the line can be determined if we also know the initialsaturation temperature of the water in the gas, and the criticalpressure at which the solid hydrate decomposes at the point of minimumtemperature. Of course this critical pressure at. which the hydratedecomposes depends on the composition of the hydrate, which depends inturn on the composition of the gas.

Assuming that'investigation has shown that with a particular gas, solidhydrate may exist in a transmission line at a point in the line wherethe temperature is 36 F. when the pressure ex-. ceeds 300 lbs. persquare inch absolute, and that the initial saturation temperataure ofthe water in the gas is 60 F., then the amount of ethanol which must beadded to each million cubic feet of gas passing through the line inorder to effectively inhibit solid hydrate formation when operating theline at a desired maximum pressure of 450 lbs. per square inch can bedetermined as approximately 7.5 lbs., and the amount of ethanol requiredfor the same purpose with an operating pressure of 800 lbs. per squareinch would be about 10.5 lbs. If methanol were substituted for ethanol,the amount required to inhibit hydrate formation in the same gas at thesame temperature would be about 10 lbs. at 450 lbs. pressure, and 13.5lbs. at 800 lbs. maximum pressure.

Minimum temperatures below 32 F. are seldom encountered in natural gastransmission lines, and such lines are, not normally operated atpressures higher than 00 lbs. per square inch. Moreover atthistemprature the natural gas which is handld by such lines does notusually form solid hydrate at pressures below about 300 lbs. per squareinch. Consequently any monohydric aliphatic alcohol with from 1 to 5carbon atoms -in its molecule will satisfactorily inhibit hydrateformation in such lines when added thereto in proportions of 2-10 lbs.of dry alcohol per million cubic feet of gas carried by the line.

Having thus described the invention, what is claimed as new is:

1. The method of inhibiting formation of solid aliphatic hydrocarbonhydrates in high pressure natural gas transmission pipe lines attemperatures above the freezing point of water, which comprises addingto the gas a volatile water soluble fluid which is without injuriouseffects upon the materials of the pipe line and in amounts sufiicient tolower the water vapor concentration in the gas below the critical valuefor hydrate formation at the lowest temperature and highest pressureobtaining in the line.

2. The method of inhibiting formation of solid hydrates of normallygaseous aliphatic hydrocarbons in high pressure natural gas transmissionpipe lines at temperatures above .the freezing point of water, whichcomprises continuously adding to the gas a small stream of a volatilewater soluble fluid which is without injurious effects upon thematerials ofthe pipe line and at a rate proportioned in accordance withthevolume of the gas so as to lower the water vapor concentration in thegas below the critical value for hydrate formation at the lowesttemperature and highest pressure obtaining in the line.

3. The method for inhibiting formation of methane hydrate in highpressure natural gas transmission pipe lines at temperatures above thefreezing point of water, which comprises continuously adding to the gasduring periods of cold weather a monohydric aliphatic alcohol havingfrom one to five carbon atoms in its molecule at the rate ofapproximately two to ten lbs. of alcohol to each million cubic feet ofgas flowing through the line. D

4. The method for inhibiting formation of methane hydrate in highpressure natural gas I high pressure natural gas transmission lines bythe formation of solid hydrocarbon hydrates in which pressures upward of300 lbs. per square inch are used, which comprises introducing arelatively small proportion of a volatile water soluble fluid which iswithout injurious effects upon the materials of the pipe line into thegas in the line during periodsof cold weather when the gas in the lineis at temperatures above the freezing point of water which wouldnormally cause the formation of hydrocarbon hydrates in the line, saidwater soluble fluid being adapted to prevent the formation of saidhydrates.

6. The method of preventing the stoppage of high pressure natural gastransmission lines by the formation of solid hydrocarbon hydratestherein from the hydrocarbons and the moisture present in the lines,which comprises introducing a relatively small proportion of a volatilewater soluble fluid into the gas flowing through the-line during periodsof cold weather when the gas temperatures are above the freezing pointof water but sufliciently low to normally cause the formation of solidhydrocarbon hydrates, said watersoluble fluid introduced into the linebeing selected from the group of fluids consisting of the aliphaticmonohydric alcohols,

ketones and acids containing from one to flve I carbon atoms inclusivein the molecule.

'7. The method of inhibiting the formation of solid hydrates of normallygaseous aliphatic hydrocarbons in high pressure natural gas transmissionpipe lines from moisture contained in the gas, passing therethrough,which comprises effecting condensation of a liquid water solution fromthe gas in the pipe line at gas temperatures above the freezing point ofwater, said condensation being efiected by continuously introducing avolatile water soluble fluid which is without injurious effects upon thematerials of the pipe line into the line at a point on the upstream sideof the line section of minimum temperature 'so that a water solution ofthe introduced fluid is formed, said fluid being introduced at a rateproportioned in accordance with the volume of gas flowing through theline.

8. The method of preventing the formation of deposits of solidhydrocarbon hydrates in high pressure natural gas transmission linesnormally caused by the formation of solid hydrocarbon injected into thegas flowing in the line in sufficient proportion to prevent theformation of deposits of solid hydrocarbon hydrates in the line.

9. The method of preventing the formation of deposits of solidhydrocarbon hydrates in high pressure natural gas transmission linesnormally caused by the formation of solid hydrocarbon hydrates thereinfrom the hydrocarbons and moisture contained in the gas beingtransmitted, which comprises introducing a relatively small proportionof a monohydric aliphatic alcohol having from one to five carbon atomsin its molecule into the gas flowing in the transmission line when thegas temperature is above the freezing point of water but is attemperature and pressureconditions which would normally cause theformation of solid hydrocarbon hydrates in the line, said alcohol beingintroduced into the gas flowing in the line in suflicient proportion toprevent formation of deposits of solid hydrocarbon hydrates therein.

10. The method of preventing the formation of deposits of solidhydrocarbon hydrates in gas transmission lines as defined by claim 9 inwhich said alcohol is methanol.

ll. The method of inhibiting formation of solid hydrates of normallygaseous aliphatic hydrocarbons in natural gas transmission linesoperated at a pressure of approximately 300 lbs.

per square inch and above at temperatures above the freezing point-ofwater, which comprises continuously adding secondary butyl alcohol tothe gas during periods of cold weather at the rate of approximately twoto ten pounds of alcohol to each million .cubic feet of gas flowingthrough the line.

BENJAMIN MILLER.

